Differencebetween Slice Assignment That Slices the Whole List and Direct Assignment

What is the difference between slice assignment that slices the whole list and direct assignment?

a_list = ['foo', 'bar']

Creates a new list in memory and points the name a_list at it. It is irrelevant what a_list pointed at before.

a_list[:] = ['foo', 'bar']

Calls the __setitem__ method of the a_list object with a slice as the index, and a new list created in memory as the value.

__setitem__ evaluates the slice to figure out what indexes it represents, and calls iter on the value it was passed. It then iterates over the object, setting each index within the range specified by the slice to the next value from the object. For lists, if the range specified by the slice is not the same length as the iterable, the list is resized. This allows you to do a number of interesting things, like delete sections of a list:

a_list[:] = [] # deletes all the items in the list, equivalent to 'del a_list[:]'

or inserting new values in the middle of a list:

a_list[1:1] = [1, 2, 3] # inserts the new values at index 1 in the list

However, with "extended slices", where the step is not one, the iterable must be the correct length:

>>> lst = [1, 2, 3]
>>> lst[::2] = []
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
ValueError: attempt to assign sequence of size 0 to extended slice of size 2

The main things that are different about slice assignment to a_list are:

1. a_list must already point to an object
2. That object is modified, instead of pointing a_list at a new object
3. That object must support __setitem__ with a slice index
4. The object on the right must support iteration
5. No name is pointed at the object on the right. If there are no other references to it (such as when it is a literal as in your example), it will be reference counted out of existence after the iteration is complete.

In python, what is the difference between slice with all the element and itself?

numbers[:] = something is known as slice assignment. It replaces the elements in the selected slice (the whole array in this case), with the elements to the right of the assignment.

numbers = something is regular assignment, which makes numbers point to something.

This example illustrates the differences:

numbers = [1, 2, 3]
something = [4, 5, 6]

numbers = something
something[0] = 10
print(numbers) # [10, 5, 6]

Notice how we may have wanted to modify the something list, but we unexpectedly modified numbers! Because they point to the same list. However, things are different with slice assignment:

numbers = [1, 2, 3]
something = [4, 5, 6]

numbers[:] = something
something[0] = 10
print(numbers) # [4, 5, 6]

numbers is still the same.

As pointed by user Tomerikoo in the comments, the slice's size doesn't have to match with whatever it's being replaced with. Which means the following is valid:

numbers = [1, 2, 3]
numbers[:] = [4, 5, 6, 7, 8, 9] 

How is this list expanded with the slicing assignment?

Slice assignment replaces the specified part of the list with the iterable on the right-hand side, which may have a different length than the slice. Taking the question at face value, the reason why this is so is because it's convenient.

You are not really assigning to the slice, i.e. Python doesn't produce a slice object that contains the specified values from the list and then changes these values. One reason that wouldn't work is that slicing returns a new list, so this operation wouldn't change the original list.

Also see this question, which emphasizes that slicing and slice assignment are totally different.

Assign value to a list using slice notation with assignee

There is indeed a difference between a[:] = b and a = b.

>>> a = [1,2,3,4]
>>> b = [4,5,6,7]
>>> c = [8,9,0,1]
>>> c = b
>>> a[:] = b
>>> b[0] = 0
>>> a
[4, 5, 6, 7]
>>> c
[0, 5, 6, 7]
>>> 

When you write a = b, a is a reference to same list as b: any change in b will affect a

when you write a[:] = b a is a list initialized with the elements of b: a change in b will not affect a

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And there also a difference between a[:] = b and a = b[:].

>>> a = [1,2,3,4]
>>> b = [4,5,6,7]
>>> c = a
>>> a = b[:]
>>> a
[4, 5, 6, 7]
>>> c
[1, 2, 3, 4]
>>> a = [1,2,3,4]
>>> b = [4,5,6,7]
>>> c = a
>>> a[:] = b
>>> a
[4, 5, 6, 7]
>>> c
[4, 5, 6, 7]

With a = b[:], you create a new list with the elements from b, if another variable pointed to a it is not affected

With a[:] = b, you change the elements of a. If another variable pointed to a it is also changed.

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